1. Field of the Invention
The invention is directed to an improved return-flow-free fuel supply system of the type employed with an internal combustion engine.
2. Description of the Prior Art
In a fuel supply system for an internal combustion engine, fuel from a fuel tank is pumped by a fuel pump via a pressure line to a fuel distributor, located on the engine and having injection valves, or to a gasoline or Diesel high-pressure pump. Modern fuel supply systems have a built-in tank unit, inserted into the fuel tank, and the fuel pump, an intake filter, and a pot as a fuel reserve are integrated with it; the pot is filled with one or more suction jet pumps. The suction jet pumps consequently assure that even when the fuel level in the fuel tank is dropping, the pot is always completely filled in order to furnish the reserve fuel. The suction jet pumps are disposed in the suction jet pump line, which branches off from the pressure line and discharges into the pot.
In modern fuel supply systems, a return line branches off from the fuel distributor and leads back into the fuel tank. The quantity of fuel not needed by the engine then flows through the fuel distributor back into the fuel tank via the return line. Conversely, in return-flow-free fuel supply systems known for instance from German Patent Disclosure DE 199 51 132 A1, no return line from the fuel distributor to the fuel tank is provided. Instead, the fuel in the fuel distributor is regulated as needed, by measuring the actual fuel pressure with a pressure sensor, comparing it inside a control unit with a set-point fuel pressure stored in a performance graph, and varying the rpm of the fuel pump as a function of the regulated difference. A check valve in the pressure line, downstream of the fuel pump, assures sealing of the pressure region that contains the fuel distributor. The regulating function is performed as long as the engine is operated under load and a quantity of fuel to be consumed is called for.
During phases in which the injection valves are closed and the fuel pump is not pumping fuel into the pressure line, for instance in a stopped phase of the engine, the pressure in the pressure line which is tight on the one hand because of the closed injection valves and on the other because of the closed check valve can rise at high temperatures, and therefore mechanically actuated pressure limiting valves or diaphragm pressure regulators are used to keep the pressure constant in the pressure line. One type of such pressure limiting valves must be constantly rinsed during operation by a slight overflow quantity, which on the one hand necessitates constant operation of the fuel pump and consequently a certain energy consumption; on the other hand, because of the slight rinsing quantity, there is the risk that dirt will become deposited on the valve seat. Another type of pressure limiting valve is closed in operation, so that after a nonpumping phase and at the onset of load operation, because of the sudden pressure rise that then occurs, both excessively rich mixtures and also, because of the higher leakage at the injection valves resulting from the pressure, higher hydrocarbon emissions can occur. Moreover, in both types of pressure limiting valves, the opening pressure cannot be varied during operation.